1. Field of Application
The present invention relates to a focus adjustment changeover circuit for facilitating the execution of focus adjustment and white balance adjustment of a projection display television receiver.
2. Prior Art Technology
With a generally utilized form of projection display television receiver, three separate cathode ray tubes (hereinafter abbreviated to CRTs) are utilized which have respective red, green and blue phosphors, for projecting respective red, green and blue television pictures. These three CRT pictures are projected onto a single display screen by projection lenses. The red, green and blue television pictures are thereby superimposed on the screen to form a projected color television picture. Generally, the emission characteristics of the phosphors of these CRTs, i.e. the relationships between CRT drive voltage and brightness of emitted light, are of the form illustrated in FIG. 1. In FIG. 1, numeral 1 denotes the emission characteristic for the green CRT and numeral 2 the emission characteristic for the red CRT, when the respective electron beams of these CRTs are precisely focused by the electron lenses of the CRTs, e.g. to produce a minimum spot size by each electron beam. Numeral 3 denotes the emission characteristic of the blue CRT under such a condition of precise focus of the electron beam, while numeral 4 denotes the characteristic of the blue CRT under a condition in which the electron beam of that tube is slightly defocused by the electron lens of the tube. This condition of slight defocusing will be referred to in the following as the defocused state. As can be seen for FIG. 1, the emission characteristic of each of the red and green CRTs is substantially linear up to high levels of drive voltage, i.e. up to high values of electron beam current. However the emission characteristic of the blue CRT exhibits a saturation condition at high levels of beam current i.e. the emission characteristic of the blue phosphor exhibits saturation at lower levels of CRT drive voltage than do the red and green phosphors. As a result, even if the white balance of the projected television picture is adjusted to be correct at some moderate level of display brightness, the white color produced on the display screen by combining light from the red, blue and green CRTs will have an insufficient level of blue component and so will have an excessively low color temperature, i.e. will not produce a pure white color, at high levels of emitted light.
However as illustrated by curve 4, this problem of saturation of the blue phosphor is alleviated to some extent if the electron beam of the blue CRT is operated in a slightly defocused condition, rather than being precisely focused. For this reason it is usual to operate the blue CRT in this slightly defocused condition, in order to provide a more accurate white balance at high levels of CRT drive voltage, and to execute white balance adjustment of the television receiver with the blue CRT in this defocused state.
This has however the disadvantage that problems arise when focusing adjustment of the projection lens of the blue CRT is executed. Specifically, this projection lens adjustment (e.g. performed while observing a crosshatch test pattern that is projected on the display screen by the blue CRT) is made difficult by the fact that the blue electron beam is defocused, and also by the fact that a projected image in blue light has inherently low visibility by comparison with a green or a red image. For this reason such a projection display television receiver is made operable in two types of adjustment modes, i.e. a mode in which electromagnetic focusing of the respective electron beams is established such that only the blue CRT is set in the defocused state and the red and green CRTs in the precisely focused state, so that white balance adjustment can be executed, and a mode in which electromagnetic focusing is established such that all of the CRTs are set in the precisely focused state, so that projection lens focus adjustment can be executed.
An example of a prior art focus adjustment changeover circuit will be described referring to FIG. 2, in which it is assumed that electromagnetic focusing of each CRT is utilized. Numeral 5 denotes an input terminal coupled to receive a video signal for producing a normal television picture, while an input terminal 6 is coupled to receive an adjustment signal which is a crosshatch test pattern signal, to produce a crosshatch test pattern picture that is utilized during focus adjustment and white balance adjustment operation. These signals from the input terminals 5 and 6 are selectively transferred to a video processing circuit 7 by a switch 8, and circuit 7 thereby produces video projection signals that are applied to drive red, green and blue CRTs (not shown in the drawings). Numerals 9, 10 and 11 denote respective static focus coils for performing static electrical focusing of the electron beams of the blue, red and green CRTs. Numerals 12, 13 and 14 denote current stabilizer circuits for supplying respective currents to the static focus coils 9, 10 and 11 respectively for static focus control. Each of the circuits 12, 13 and 14 is configured as shown for current stabilizer circuit 12, i.e. including a pair of transistors 15 and 16 which are connected in common-emitter configuration, and drive and feedback transistors 17 and 18.
An input terminal 19 is coupled to receive a parabola waveform voltage signal during each vertical scanning period, for vertical focus control. This signal is amplified by a transistor 20, and is then adjusted in amplitude by means of a potentiometer 21 and transferred through respective capacitors of a capacitor group 22 to the bases of the respective transistors 15 of the current stabilizer circuits 12, 13 and 14. In this way, electron beam focus deviations between the center and the upper and lower regions of the projected picture are corrected.
Numerals 23, 24 and 25 denote respective potentiometers which are utilized for static focus adjustment of the blue, green and red CRTs respectively. DC voltages derived from these potentiometers, adjusted to provide appropriate levels of electromagnetic focus currents, are applied through respective ones of a set of resistors 26 to the bases of the respective ones of transistors 15 of the current stabilizer circuits 12, 13 and 14. Static focus adjustment of the electromagnetic focus currents of the focus coils 9, 10 and 11 of the blue, green and red CRTs can thereby be mutually independently executed.
Focus adjustment of a projection display television receiver by utilizing such a prior art apparatus is executed in two stages. In a first stage, the changeover switch 8 is set to supply the crosshatch test pattern signal from the input terminal 6 to the video processing circuit 7. A crosshatch test pattern is thereby projected on the display screen of the television receiver by the CRTs. In this condition, the potentiometers 23, 24 and 25 are respectively adjusted such as to vary the levels of DC voltage applied to the bases of the transistors 15 of the current stabilizer circuits 12, 13 and 14, to set the levels of current flow in each of the electromagnetic focus coils 9, 10 and 11 such as to precisely focus the electron beams of each of the blue, green and red CRTs. When this has been completed, optical focusing of the respective projection lenses of the blue, green and red CRTs is executed to attain an optimum focus condition of each of the blue, green and red images projected on the display screen.
In the second stage, the changeover switch 8 is set such as to supply the signal from input terminal 5 to the video processing circuit 7, for thereby projecting a television picture. The focus adjustment potentiometer 23 for the blue CRT is then adjusted such as to slightly defocus the electron beam of the blue CRT. White balance adjustment of the projection display television receiver is then executed, with the color temperature of the projected picture at high levels of brightness being improved due to the slightly defocused state of the electron beam of the blue CRT as described hereinabove.
Each time that the installation conditions of a projection display television receiver are altered, (i.e. when the television receiver is moved to a different location, or the distance to the display screen is altered, etc) it is necessary to repeat the two steps described above, in order to first adjust the focus of the projection lenses and then re-adjust the white balance. However with such a prior art focus adjustment circuit, each time that these two steps must be repeated in this way, it is necessary to first adjust the potentiometer 23 for changing the electromagnetic focusing of the electron beam of the blue CRT from the defocused state described above to the precisely focused state, then to execute focusing of the projection lenses, then to again adjust the potentiometer 23 to return the potentiometer 23 to a position which provides the defocused state of the blue CRT. It is therefore necessary to frequently make delicate adjustments to the electromagnetic focus potentiometer 23. Furthermore since such adjustment of the potentiometer 23 is executed manually, fixedly predetermined adjustment statuses cannot be ensured, so that it is difficult to establish the defocused state and the precisely focused state for the blue CRT each time that such an adjustment procedure is performed.